Open Seminar by Professor Karl Kainer
K.U. Kainer, D. Tolnai, Y. Huang, R.H. Buzolin, and N. Hort
Helmholtz-Zentrum Geesthacht, Institute of Materials Research, Magnesium Innovation Centre, Max- Planck-Str. 1, 21502 Geesthacht, Germany
Magnesium alloys are multi-purpose materials and possess properties which entitle them as future material of choice in some of the vital key technologies. Their light weight and excellent specific strength makes them attractive for the transportation and energy sectors. Furthermore their in-vivo corrosion properties unequivocally qualify particular Mg alloys to be used as smart materials in the field of bio- degradable implant development. These applications all require a very specific property profile, which can be obtained through the optimization of processing routes and microstructure design. Nowadays, several factors hinder the further development and the widespread usage of Mg alloys, e.g. the low formability and yield strength at ambient temperature and the strong degradation of mechanical properties above 200 °C. These obstacles can be overcome by understanding the characteristics of phase-formation and -evolution during solidification and processing, which is the first step towards microstructure engineering.
The high brilliance of the available beam in synchrotron sources allows for fast acquisition times and performing experiments with the necessary temporal resolution. Tomography provides information about the evolution of the internal architecture e.g. how the dendritic structures grow and where the sites for the formation of the secondary phases are. The diffraction data gives insight to the solidification path e.g. which phases solidify at which temperature, if there is meta-stable phase that transforms during cooling and if the cooling rate has an effect on the phase-formation and –transformation. During processing azimuthal angle – time plots can be constructed from the 2D Debye-Scherrer patterns taken during the in situ deformation experiment. These plots give information on grain size evolution, grain imperfection, grain rotation, and grain orientation, texture evolution, strain and strain anisotropy. This information can be correlated with dislocation slip, sub-grain formation, twinning, recovery, recrystallization and grain growth.
The disadvantage of the limited sampling volume originating from the cross section and the penetration depth of the synchrotron beam can be overcome by neutron diffraction. The effects of alloying elements on the deformation of magnesium were discussed, especially the role of RE in weakening the texture was evaluated. With these techniques, an unexpected hydride REH2 was identified in RE-containing alloys. Furthermore, neutron technique was also employed to analyze the residual stress near the hot sprue in Mg-Zn castings. The results showed that the increase of Zn content decreases the residual stress in the hot sprue region.